system: Add and update Timer requirements

Add new Timers system requirements.
Remove user story from the system requirements.
Update description and titels of the existing requirements to
fit better to the guidelines.

Signed-off-by: Simon Hein <[email protected]>

Author: simhein

docs/system_requirements/index.sdoc

@@ -268,18 +268,20 @@ As a Zephyr RTOS user, I want to be able to give my threads different priorities
 
 [/SECTION]
 
+[SECTION]
+TITLE: Timers
+
 [REQUIREMENT]
 UID: ZEP-SYRS-18
 STATUS: Draft
 TYPE: Functional
 COMPONENT: Timers
-TITLE: Timers
+TITLE: Time based events
 STATEMENT: >>>
 The Zephyr RTOS shall provide a framework for managing time-based events.
 <<<
-USER_STORY: >>>
-As a Zephyr RTOS user, I want to start, suspend, resume and stop timers which shall trigger an event on a set expiration time.
-<<<
+
+[/SECTION]
 
 [REQUIREMENT]
 UID: ZEP-SYRS-19